The human visual system consists of three primary components, the cornea, the crystalline lens and the retina. Emmetropia is the condition in which incoming parallel light rays focus perfectly on to the retina so that clear images i.e. 20/20 vision, will be seen. In myopia, or nearsightedness, parallel light rays are focused anterior to the retina resulting in a blurred retinal image. In hyperopia or farsightedness, the parallel light rays focus posterior to the retina again resulting in an out of focus image. Other refractive anomalies such as astigmatism and presbyopia also result in blurred retinal images.
Corrective modalities for the above mentioned refractive errors include spectacles, contact lenses and refractive surgery. Spectacle lenses have been the traditional mode used to correct refractive errors and they consist of concave, convex or cylindrical lenses to bring the unfocussed parallel rays of light to focus on the retina. Contact lenses accomplish a similar optical correction by the placement of a rigid or soft plastic material directly onto the surface of the cornea. Surgical correction of refractive errors (i.e. RK, PRK, LASIK, intraocular lenses) works by either remodeling the surface of the cornea or by addition of a corrective lens surgically implanted within the eye. In 1962, Jessen introduced a non-surgical contact lens technique for remodeling the corneal surface which was eventually referred to as orthokeratology.
Traditionally, orthokeratology has been defined as the temporary reduction or elimination of refractive errors (myopia, hyperopia, astigmatism and presbyopia) through the application of specially designed rigid gas permeable lenses (RGPs) to reshape the anterior surface of the cornea. The desired topographical changes (central corneal flattening to correct myopia and central corneal steepening to correct hyperopia) are made possible through a unique posterior lens geometry in which the center of the lens incorporates a different radius of curvature than the mid-periphery. One such geometry that is commonly employed is referred to as a “reverse geometry design”. In contemporary orthokeratology, the posterior lens configuration reshapes the corneal surface overnight while the patient is sleeping. Upon awakening, the lenses are removed and the patient experiences reduced refractive error and improved vision without glasses, contact lenses, or refractive surgery. Since the corneal remodeling is not permanent, it is necessary for the patient to wear the rigid contact lenses every night or every other night to retain the desired effect.
All previous orthokeratology technologies have used hard lenses or RGP lenses to achieve the corneal reshaping effects.
While research is continuing into the mechanism underlying RGP orthokeratology, current understanding is that a hydraulic, tissue-altering force is generated beneath a rigid lens in which a significant differential in tear volume may be present.
In the case of myopic orthokeratology, the desired central flattening effect is made possible through an RGP lens that incorporates a central radius of curvature that is flatter (ie larger radius of curvature) than the curve of the central cornea. In the mid-periphery, the lens incorporates a radius of curvature that is steeper (ie small radius of curvature) than the curve of the cornea. Together, these curves combine to form the basis of a reverse geometry lens design.
The posterior shape of a reverse geometry lens creates a positive “push” force on the center of the cornea by virtue of a thin (approximately 5 micron) tear layer across the central cornea. The steeper mid-peripheral curve of the lens creates a thick tear layer (approximately 550 microns) resulting in a negative pressure or “pull” force. This negative pressure leads at least in part to a relative increase in mid-peripheral corneal thickness with respect to central thickness. Together, these two forces create the desired changes seen in myopic orthokeratology.
In the case of hyperopic orthokeratology, the current understanding is that the mechanism works opposite to that of myopic orthokeratology. In other words, the lens design creates a “pull” (negative) pressure in the center and a “push” (positive) pressure in the mid-periphery. These forces may be generated by a reverse geometry lens design that incorporates both a steep central radius of curvature and a flat mid-peripheral radius of curvature. This configuration creates the desired changes seen in hyperopic orthokeratology.
Currently, there is a wide range of lens designs marketed for corneal reshaping (Table 1). In the United States, at least one design, the Paragon CRT, has been FDA approved for overnight corneal reshaping. The remaining lens designs are either approved for daily wear only or currently in some phase of their clinical studies for overnight FDA approval. All of the lenses in Table 1 are RGP lenses.
TABLE 1A number of lens designs marketed for corneal reshaping.Lens DesignManufacturerCorneal Refractive TherapyParagon Vision SciencesBE DesignPrecision TechnologyContex E SystemContexDreimLensReimLens Inc.Emerald DesignEuclid SystemsNightFormCorrectechControlled Kerato ReformationSami El HageR&R DesignRinehart/ReevesNightMoveRoger TabbFargo DesignJim DayOrthoFocusMetro OpticsWave SystemCustom CraftReversible Corneal TherapyABBA OpticalFree Dimension/e LensE and E OpticsAlignment Series/FalconG.P. Specialist
The Paragon CRT lens consists of three primary zones. The first zone consists of a central base curve radius designed to correct myopic refractive error. This flatter radius of curvature is instrumental in creating the appropriate forces beneath the lens to facilitate the remodeling of corneal tissue. The second zone, the return zone, is a sigmoid shaped curve that controls the amount of lens clearance across the central cornea. A shallower sigmoid curve brings the base curve into closer apposition to the cornea, whereas a deeper sigmoid curve results in greater apical clearance. The third and final zone provides alignment of the lens across the mid-peripheral cornea. This zone terminates in a controlled edge curve designed to maximize patient comfort.
For many years, RGP lenses were the physiologically preferred lenses for most contact lens wearers. This was because RGP lenses have high levels of oxygen transmissibility, and are generally considered to be relatively physiologically non-damaging to the wearer's eye, for example through the greater tear exchange achievable. RGP lenses are not, however, particularly comfortable to wear, and more recently soft contact lenses have become the lens of choice for most patients. Indeed, in some countries, approximately 90% of contact lens wearers now use soft lenses. Silicone hydrogel lenses are a relatively recent development, and provide high levels of oxygen transmissibility thereby eliminating the previous disadvantage of soft contact lenses and permitting safe overnight wear while retaining a high comfort level. Most recently, silicone hydrogel extended or continuous wear lenses have been developed which have sufficient tear and oxygen transmissibility so as not to cause damage to the eye, even when the lenses are worn overnight during sleep, or even continuously for up to 30 days.
It will be appreciated that soft lenses tend to conform far better to the shape of the wearer's eyes than do RGP lenses. Indeed, it is the softness and conformability of soft lenses that is believed to provide high comfort levels for the wearer. The orthokeratology process requires some reshaping of the surface of the eye, and accordingly it has been accepted wisdom that soft lenses, because of the high degree of conformity to the surface of the eye, would be unsuitable for orthokeratology.